Many digital cameras operate using an array of photosensitive elements, such as photodiodes, that generate an electrical signal in response to photons impinging on the sensitive surface of the photodiode. Charge-coupled devices (CCDs) may be coupled to the photosensitive elements to transfer the electrical signals (e.g., charge) from the photosensitive array. In the context of image acquisition, CCDs form and operate as shift registers capable of passing along charge to neighboring CCDs. To transfer the charge stored by the photodiode array, CCDs may be arranged to transfer charge vertically through vertical shift registers and horizontally through a horizontal shift register to serialize the charges stored by the array.
FIG. 1 illustrates a conventional method of transferring charges stored by a photosensitive array. Array 100 may be an array of photosensitive elements, such as photo-diodes, arranged in horizontal rows (e.g., rows A-J) and vertical columns (e.g., columns 1-10). Vertical CCDs are arranged to transfer charge from the photosensitive array into vertical shift registers that shift charge vertically (e.g., in a direction from row A to row J). A horizontal shift register 120 may be arranged to transfer charge horizontally (e.g., in a direction from column 1 to column 10). Thus, on each shift of the horizontal shift register 120, the charge currently stored at the last element of the register (e.g., at the J10 side of the horizontal register) is transferred out of the register and ultimately stored as a digital pixel value. The charge is typically converted into a voltage signal, amplified and sampled to obtain digitized values for the stored charge. As a result, the charges stored on the array are serialized into a time-varying voltage signal which may be digitized and stored as a digital image.
To avoid overwriting charges stored in the horizontal shift register, the horizontal shift register may need to be operated at a higher frequency. For example, all the charges in horizontal shift register may need to be transferred out before the vertical shift registers shift their respective last element charges into the horizontal shift register. Accordingly, in FIG. 1, the horizontal shift register must shift ten times for each vertical shift of charges. As a result, the horizontal clock may be operated at a much higher frequency than the vertical clock in order to properly transfer all of the charge off the array.
It should be appreciated that array 100 is exemplary and used merely to illustrate one method by which charges are transferred off an array of photosensitive elements. Typical photosensitive arrays will be much larger, as will be understood by those of ordinary skill in the art. In addition, the method described above is simplified to illustrate schematically how charges may be transferred from the photosensitive array. The terms vertical and horizontal are used arbitrarily to denote any two directions used to serialize two-dimensional data (e.g., a two-dimensional array of charges). However, charge may be transferred horizontally and then vertically as well.
Moreover, the use of the terms vertical and horizontal are used to be consistent with the terminology used in the art to describe the “direction” of charge transfer. In practice, charge transfer may not occur vertically and horizontally. For example, charge may be transferred down into the semiconductor device via shift registers and then transferred through another shift register to be serialized. Accordingly, the term vertical refers herein to a first direction of charge transfer and horizontal refers herein to a second direction of charge transfer. The terms vertical shift register and horizontal shift register are used herein to distinguish between separate shift registers and do not otherwise limit the nature of the shift registers, or elements of the shift register.